This invention relates to a field effect transistor comprising a heterojunction structure for use in generating a two dimensional electron gas which is used as a carrier.
A field effect transistor of the type described, comprises a first semiconductor layer and a second semiconductor layer formed on the first semiconductor layer. A heterojunction structure is formed between the first semiconductor layer and the second semiconductor layer. Such a field effect transistor may be called a high electron mobility transistor.
In order to improve a transistor characteristic of the field effect transistor, it is necessary to make a conduction-band discontinuity be high between the first semiconductor layer and the second semiconductor layer.
A conventional field effect transistor of the above-mentioned type is disclosed in a title of "High Transconductance InGaAs/AlGaAs Pseudomorphic Modulation-Doped Field-Effect Transistors" contributed by A. Ketterson et al to IEEE ELECTRON DEVICE LETTERS, Vol. EDL-6, December 1985, pages 628 to 630.
In the conventional field effect transistor, an undoped InGaAs semiconductor layer is used as the first semiconductor layer. An n-type AlGaAs semiconductor layer is used as the second semiconductor layer. According to a report contributed by S. W. Corzine et al to APPLIED PHYSICS LETTERS, Vol. 57, No. 26, 1990, pages 2835-2837, it is known that the conventional field effect transistor has the conduction-band discontinuity of 0.28 eV when the undoped InGaAs semiconductor layer is composed of In.sub.0.2 Ga.sub.0.8 As and the n-type AlGaAs semiconductor layer is composed of Al.sub.0.2 Ga.sub.0.8 As.
However, it is difficult to obtain a high transistor characteristic in the conventional field effect transistor inasmuch as the conventional field effect transistor does not have enough of the conduction-band discontinuity between the first semiconductor layer and the second semiconductor layer.